In a small teahouse in the conservative southeastern Turkish town of Adiyaman, two brothers would hold court with young followers, praying, reading the Koran and painting a picture of a better life across the border in Syria, in the ranks of Islamic State.

In a small teahouse in the conservative southeastern Turkish town of Adiyaman, two brothers would hold court with young followers, praying, reading the Koran and painting a picture of a better life across the border in Syria, in the ranks of Islamic State.

Now the Islam Tea Shop, its upper windows blacked out and etched with white Arabic script, has closed down and the brothers, according to authorities, are both probably dead; one behind a July suicide attack that killed 33, the other a prime suspect in Saturday's Ankara bombing that claimed 99 lives.

In barber salons and grocery stores, talk is of the Alagoz brothers, anxious accounts of what they might have said and done in the Islam Tea Shop, how they went undetected; and how their town of 200,000 has assumed a national notoriety as home to what authorities call the "Adiyaman Cell" of Islamic State.

Adiyaman, though clearly poor and offering scant employment to the young, is an unremarkable Anatolian town of narrow streets, kebab houses, small shops and tea houses where men linger over backgammon. It is pious, but not intolerant, with no outward sign of the militancy that marks the Islamist militants.

Demonstrators hold a banner depicting Turkish President Tayyip Erdogan (R) and bombing casualties, during a march to comemorate the victims of Saturday's bombing of an Ankara rally of pro-Kurdish activists and civic groups, through the streets of Frankfurt, Ocotber 14, 2015. REUTERS/Kai Pfaffenbach

Adiyaman sheds light on the vulnerability of a country not otherwise known for religious militancy. And the stability of Turkey, a NATO member which serves as a buffer against turmoil in nearby Syria, has a significance well beyond its borders.

Several families say they approached authorities expressing concern their relatives were embracing radical Islam and may have gone to Syria. Others tried to confront the brothers as the group became increasingly withdrawn, avoiding local mosques and holding Friday prayers at the shop on a small side street.

Demonstrators hold a banner depicting Turkish President Tayyip Erdogan (R) and bombing casualties, during a march to comemorate the victims of Saturday's bombing of an Ankara rally of pro-Kurdish activists and civic groups, through the streets of Frankfurt, Ocotber 14, 2015. REUTERS/Kai Pfaffenbach

Among them was the family of Orhan Gonder, now in jail pending charges for the June bombing of a pro-Kurdish rally in the southeastern town of Diyarbakir.

"One day we went to the tea shop and had an argument with the guys," Orhan's cousin Ercan told Reuters. "We yelled at them, saying 'why are you brainwashing our kids? Stop interfering with their lives'."

Demonstrators carry mock coffins to comemorate the victims of Saturday's bombing of an Ankara rally of pro-Kurdish activists and civic groups, as they march through the streets of Frankfurt, Ocotber 14, 2015. REUTERS/Kai Pfaffenbach

"They were very calm and confident, we couldn't intimidate them at all, they said 'we don't harm anyone. We just read the Koran here and they come and sit with us voluntarily'."

Now their faces and those of others from the town are emblazoned across national newspapers.

Demonstrators carry mock coffins to comemorate the victims of Saturday's bombing of an Ankara rally of pro-Kurdish activists and civic groups, as they march through the streets of Frankfurt, Ocotber 14, 2015. REUTERS/Kai Pfaffenbach

Authorities say that in July, one of the Alagoz brothers, Seyh Abdurrahman Alagoz, walked into a group of pro-Kurdish student activists in Suruc near the Syrian border and blew himself up.

His brother, Yunus Emre Alagoz, is a prime suspect in the Ankara bombing, the worst attack of its kind in the history of modern Turkey.

Demonstrators carry mock coffins to comemorate the victims of Saturday's bombing of an Ankara rally of pro-Kurdish activists and civic groups, as they march through the streets of Frankfurt, Ocotber 14, 2015. REUTERS/Kai Pfaffenbach

The two Ankara blasts, three weeks ahead of an election, triggered protests against what critics of President Tayyip Erdogan and the government see as major intelligence failings.

Prime Minister Ahmet Davutoglu, highlighting the complexity of radical groups confronting Turkey, told Reuters both Islamic State and Kurdish militants could have played a role in the Ankara suicide attack.

Demonstrators carry mock coffins to comemorate the victims of Saturday's bombing of an Ankara rally of pro-Kurdish activists and civic groups, as they march through the streets of Frankfurt, Ocotber 14, 2015. REUTERS/Kai Pfaffenbach

Two officials at the Adiyaman prosecutor's office, who declined to be named because they are not authorised to speak to the media, told Reuters the Alagoz brothers' father had filed a criminal complaint against Yunus on Oct. 15, 2014 on suspicion that he had joined an "armed terrorist organisation".

Reuters was not able to contact the Alagoz family. Several local residents said they had left Adiyaman in July after the Suruc bombing, while a small grocery shop in the town centre said to have been owned by another brother was closed.

People look at the site of deadly Saturday explosions in Ankara, Turkey, Wednesday, Oct. 14, 2015. (AP Photo/Emrah Gurel)

Adiyaman's chief prosecutor declined to comment because the investigation was in process.

"Gonder and Alagoz, both Kurdish, went to Syria together in October 2014 to the town of Kobani with the same group," one senior government official said, referring to a town near the Turkish border which at that time was under siege by Islamic State.

"The political climate (in Adiyaman) was quite conducive to radicalisation. Conservative Kurdish youth were marginalised."

Adiyaman is more conservative than other parts of Turkey's predominantly Kurdish southeast, where many Kurds were incensed by Ankara's reluctance last year to help their brethren in Syria defend Kobani against an Islamic State siege.

A Kurdish insurgency, launched in 1984, has cost over 40,000 lives. It has entered a new, more pernicious stage since the collapse of a two-year-old ceasefire in July, presenting Turkey effectively with a war on several fronts - with Kurdish fighters, with Islamic State and with leftist militants.

The opposition CHP, in a report written after a visit to Adiyaman earlier this year, said that based on what residents have been telling them it believed 300-500 people may have joined Islamic State from Adiyaman - a number that many might consider rather high.

Gonder and Alagoz were not the only families in Adiyaman to report young male relatives to the authorities in recent years, fearing they had left to join Islamic State in Syria.

Investigators asked some to provide blood for forensic testing in the days after the Ankara bombing, which targeted a rally of leftist pro-Kurdish activists and civic groups.

Retired Adiyaman municipal worker Hikmet Kizilbay, 53, and his wife were among those asked to give blood. Their son, Haci, was identified in Turkish newspapers as among 21 people being investigated over the Ankara blasts, 18 of them from Adiyaman.

Kizilbay said he reported his son to police two years ago after he disappeared, abandoning plans to go to university. He returned some seven months later and tried to take his mother's gold bracelets, accusing her of being an infidel and saying he wanted to give money to "our Muslim brothers fighting in Syria".

"He's gone, they brainwashed him," Kizilbay told Reuters, fighting back tears as he sat in the car he now uses to earn money as a private taxi driver.

"Even if he comes back again, he's not the same person any more ... If he's going to harm our society, kill our soldiers, police, teachers and students, it's best he comes back in a coffin," Kizilbay said.

The possibility that a group known to the authorities carried out the Ankara bombing has heaped pressure on the government, already under fire from opponents for failing to give more transparent information on its investigations into the Diyarbakir and Suruc attacks.

Prime Minister Davutoglu said on Wednesday an investigation was underway into possible intelligence failures and that "any negligence" would be uncovered.

But he said it was difficult for Turkey, which has taken in more than 2 million Syrian refugees, to monitor every single individual crossing the 900 km (560-mile) border.

Ercan Gonder said his cousin Orhan had begun behaving in a bizarre way in early 2014.

"He refused to sit with his female cousins at the dining table, he started praying fives times a day, he began wearing loose clothes," Gonder said.

"It never occurred to us he would go and join Islamic State, frankly. But in October 2014 he began going to the tea shop more and more."

Osman Suzen, head of the local branch of Turkey's Human Rights Association said the brothers saw Adiyaman as a prime territory for recruitment.

"The driving force was their social ties. These boys knew each other, they believed in each other and that's how they convinced each other. All these people, their names are present in the state's records but until this day no serious investigation has been launched."